1. Field of the Invention
The present invention relates to a working method by a laser and, more particularly, to a technique in which a hole digging work of a copper foil pattern of a printed circuit board (hereinafter, referred to as a print board) is executed by a laser.
2. Related Background Art
The inventors of the present invention have already proposed a technique in which in order to work through holes into a print board, lasers of different kinds of wavelengths are irradiated onto a working surface of the print board and holes are worked in Japanese Patent Application No. 1-3462.
On the other hand, in JP-A-62-289390, there has been disclosed a laser working machine in which two laser beams of different wavelengths are used and the beams are irradiated to the same location to thereby work.
Further, in JP-A-62-254117, there has been disclosed an apparatus in which different kinds of laser beams which are oscillated from independent laser apparatuses are mixed and irradiated.
Hitherto, when a micro through-hole is formed, a hole forming work is executed by using a drill having a microdiameter of, for instance, about 0.2 to 0.3 mm.
In the case of the hole forming work using such a drill, when the number of holes to be formed is small, a high working accuracy of a certain degree is obtained. However, when the hole forming work is continuously executed, the cutting performance deteriorates and the drill is damaged due to heat generation during the working operation of the drill, so that the working time becomes long and the surface roughness of the cross section of the hole also deteriorates. On the other hand, in the case of the drill working, the drill having a diameter of about 0.1 mm is easily bent or damaged and it is also difficult to automatically execute the hole forming work.
Further, in the case of the above drill working, the surface roughness of the worked cross section becomes very rough and even if the front and back surfaces are plated in order to make the copper foil patterns on the front and back surfaces conductive after the holes were formed, an enough plating coated film cannot be obtained on the cross sectional surface.